• When magma reaches the surface it loses its gases and

become lava

• The igneous rocks that form on the surface are called

extrusive or volcanic (after the fire god Vulcan).

• Magmas that fail to reach the surface crystallize below

the surface and produce the intrusive or plutonic rocks

(after Pluto, the god of the lower world).

• Plutonic rock would only appear on the surface of the

Earth when crust is uplifted and the overlying rocks are

eroded. The exposures of these rocks are called outcrops

Igneous Rocks

Formed by cooling and

hardening of magma

•Plutonic –

intrusive

forms underground

•Volcanic –

extrusive

forms on surface

Two kinds of magma

• High SiO2%, light

colored, thick, slow

moving

•Low SiO2%, dark

colored called mafic

Igneous Rocks

•Grain size and

texture depends on

how quick cooled

–Slow = large

crystals

–Fast = small crystals

Igneous Rocks

•Grouped based on

mineral composition

–Light = granite

–Dark = gabbro

Igneous Compositions

Granite: quartz + alkali feldspar

Coarse grained = phaneritic

Granites intruded by dikes

NE Aqaba

Rhyolite (quartz + alkali

feldspar)

fine-grained = aphanitic

Andesite porphyry

two distinct sizes

Scoria

Pumice

Obsidian

Diorite

Porphyritic granite